12. Alkenes and Alkynes

Question 1

Alkene

Answer 1

-unsaturated hydrocarbon that contains C-C double bond
-bond angle: around 120°
-free rotation in C-C double bond does not normally take place

Question 2

Alkyne

Answer 2

-unsaturated hydrocarbon that contains C-C triple bond

Question 3

Ethylene

Answer 3

-simplest alkene
-trace amounts in nature; large artificial production
-natural ripening agent for fruits
-fruits can be treated with ethylene gas or ethephon (ethrel) which releases ethylene

-from thermal cracking of ethane

Question 4

Thermal cracking

Answer 4

-saturated hydrocarbon is converted to unsaturated hydrocarbon plus H2

Question 5

Cis-trans isomers

Answer 5

-isomers that have same connectivity of their atoms but different arrangement in space
-results from the presence of either a ring or C-C double bond
(i.e. cis-2-butene and trans-2-butene)

Question 6

IUPAC naming
ALKENES

Answer 6

1. Longest carbon chain that includes double bond. Indicate length of parent chain using prefix
2. Number chain to give lowest number to carbon atoms of double bond. Designate the position of double bond by the number of its first carbon
3. Branched alkenes are named in similar manner to alkanes
   (i.e. 4-methyl-1-hexene; 2,3-diethyl-1-pentene)

Question 7

IUPAC naming
ALKYNES

Answer 7

-similar lang sa alkenes except -yne suffix
-when both double and triple bonds are present, lowest number is given to the bond that comes first
-“–en” suffix follows the parent chain. Location of double bond is indicated before the parent name; location of triple bond is indicated between “–en” and “–yne”

(i.e. 3-penten-1-yne;
3,4-dipropyl-1,2-hexadien-5-yne;
1,4,4-trimethylcyclobutene)

Question 8

Common names

Answer 8

-usually for low molecular weights

Ethene : Ethylene
Propene : Propylene
2-Methylpropene : Isobutylene

Ethyne : Acetylene
Propyne : Methylacetylene
2-Butyne : Dimethlyacetylene

Question 9

Cis and trans configurations of alkenes

Answer 9

-CIS: carbons of parent chain are on the same side of double bond
-TRANS: opposite sides

Question 10

Cycloalkenes naming

Answer 10

-number double bond in 1 and 2 in the direcrion that gives substituent encountered first the lowest number
-not necessary to indicate location of double bond since matic 1 and 2

-i.e. 4-Ethyl-1-methylcyclohexene

Question 11

Dienes, Trienes, Polyenes

Answer 11

-Containing more than one double bond: alkadienes, alkatrienes, polyenes

i.e. 2-Methyl-1,3-butadiene
1,3-Cyclopentadiene

-for alkene with “n” C-C double bonds, each can show cis-trans isomerism of:
2^n stereoisomers

Question 12

Physical properties of alkenes and alkynes

Answer 12

-non polar; London dispersion only
-similar to alkanes having same carbon skeletons
-less dense than water (float)
-soluble to non polar

Question 13

Terpenes

Answer 13

-compound whose carbon skeleton can be divided into two or more units identical to the five-carbon skeleton of isoprene (2-methyl-1,3-butadiene)
-found in essential oils of plants

Question 14

Isoprene unit

Answer 14

-carbon 1: head
-carbon 4: tail
-tail of one isoprene unit becomes bonded to the head of another isoprene unit

Question 15

Characteristic reactions of alkenes

Answer 15

-addition to C-C double bond
-double bond brooen; replace by single bonds form to two new atoms or group of atoms

Question 16

Addition of hydrogen halides (HYDROHALOGENATION)

Answer 16

-HCl, HBr, HI
Alkene + hydrogen halides ➡️ alkyl halides

Question 17

Regioselective reaction

Answer 17

-reaction in which one direction of bond forming or bond breaking occurs in preference to all other directions

Question 18

Vladimir Markovnikov

Answer 18

-Markovnikov’s rule:
In the addition of HX or H2O to an alkene, hydrogen adds to the carbon of the double bond having the GREATER NUMBER OF HYDROGEN
“the rich get richer”

Question 19

Reaction mechanism

Answer 19

step by step description of how a chemical reaction occurs

Question 20

Carbocation

Answer 20

-species containing a carbon atom with only three bonds and bearing positive charge

Question 21

Addition of water: Acid-catalyzed reaction
(HYDRATION)

Answer 21

-catalysis: usually H2SO4
Alkene + H2O ➡️ Alcohol
-follows Markovnikov’s rule

Question 22

Oxonium ion

Answer 22

-ion in which oxygen is bonded to three other atoms and bears a positive charge

Question 23

Addition of Bromine and Chlorine
(HALOGENATION)

Answer 23

-addition of halogens to carbon atoms of double bond

Question 24

Addition of Hydrogen: Reduction
(HYDROGENATION)

Answer 24

Alkenes + H2 ➡️ Alkanes
-in presence of transition metals (Pt, Pd, Ru, Ni)
-aka catalytic reduction/ catalytic hydrogenation

Question 25

Oxidation

Answer 25

Gain oxygen
Lose hydrogen

Question 26

Reduction

Answer 26

Lose oxygen
Gain hydrogen

Question 27

Polymer

Answer 27

-Greek: poly and meros; many parts
-any long chain molecule synthesized by bonding together many single parts called monomer

Question 28

Mono

Answer 28

-simplest nonredundant unit from which a polymer is synthesize

Question 29

Chain-growth polymers

Answer 29

-single most important reaction of alkenes in chemical industry
-alkene form polymers made by stepwise addition of monomers

Question 30

Peroxide

Answer 30

-any compound that contains —-O—-O— bond
-i.e. Hydrogen peroxide
H—O—O—H

Question 31

Low-density polyethylene (LDPE)

Answer 31

-first commercial process for ethylene used peroxide initiators at 500°C and 1000 atm
-yielded to LDPE
-tough and transparent
-highly branched
-used for manufacture of films (for packaging baked goods and vegetables, trashbags)

Question 32

High-Density Polyethylene (HDPE)

Answer 32

-alternative method for polyrization of alkenes
-do not use peroxide initiators
-Karl Ziegler and Giulio Natta (1950s)
-yielded HDPE
-little branching; packs more closesly together
-stronger van der waals than HDPE
-milk and water jugs, grocery bags, squeezable bottle